Solar Hot-Water Heaters: Green and Hot H2O

by Marshall

Using the sun's energy to heat water isn't a new idea. The Romans warmed their baths with large south-facing windows and modern-prototype solar water-heating systems of copper tubes in glass-enclosed boxes were invented by the late 1800s. But these pioneering forays seemed to be lost on Americans until the 1970s, when oil shortages prompted Jimmy Carter to grant generous federal and state solar-energy tax credits for solar water heaters.

The tax credits accomplished their goal; they stimulated sales for solar thermal systems and created a burgeoning industry almost overnight. Unfortunately, though, they also bred a wanton lust for the tax credits themselves -- an urge than often seemed to overpower the impetus for good solar energy. The newborn industry was riddled with unscrupulous solar businesses. Perhaps wiser legislation or time and marketplace forces would have sorted this out, but the experiment was abruptly ended when Ronald Regan eliminated the tax advantages. Although this devastated the solar industry, it also flushed out the overnight experts and tax-credit hucksters. In the long run, the solar industry is stronger for it, since it's developed increasingly reliable and efficient products serviced by seasoned professionals. (If only we could apply such laissez-faire economic policies to the oil industry and remove their subsidies.)

Why Consider a Solar Water Heater?

Keeping hot water on tap is the second largest household energy user there is after space heating and cooling. So it's smart to be smart about how we handle it. Over its 10 to 13 year life, the average 60-gallon electric hot-water tank serving a family of four will cost $5,500 to $7,000 in addition to its $350-plus initial "cheap" price tag. And obviously, after that decade or so is up, it will need to be replaced by another one.

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According to Ken Olson, solar-thermal editor for California-based Home Power Magazine, "An investment in a solar water-heating system will beat the stock market any day, any decade, risk-free. Initial return on investment is on the order of 15 percent, tax-free, and goes up as gas and electricity prices climb. Many states have tax credits and other incentives to sweeten those numbers even more." (To check out your state's incentives, see www.dsireusa.org.)

Olson's financial assessment, while probably appropriate for much of the Sunbelt, doesn't apply to New England's climate or if you're already using an efficient gas water heater and careful about hot-water use. But investing is still a good way to think of buying a solar water heater, both because they're good for the planet and because their higher up-front cost compared with conventional water heaters requires that we consider their financial benefits over the years.

Costs for a solar hot-water system appropriate for the Northeast can vary widely, depending on the type of system, whether you install it yourself or buy used components, and how much hot water you want it to generate. At the low price end, if you built a "simple," seasonal, supplemental batch water heater yourself (explained below), you could manage on $300 to - $500. While a soup-to-nuts installed system that takes care of 70 to 80 percent of your hot -water usage could run as much as $9,000 to $11,000. Obviously this entails a much higher initial cost compared with conventional water heaters, but it's good to keep in mind that solar systems can last 30 to 50 years (or some say indefinitely, if well maintained). So the longer you have your heater, the better it works out as a financial investment (and hedge against inflation -- assuming electric prices rise over time).

As an investment in the earth, a solar system pays off as soon as it is installed, since it produces pollution-free hot water. Using Department of Energy data, replacing an electric water heater with a solar one saves 86,400 pounds carbon dioxide emissions over 20 years--roughly the equivalent of taking a fuel-efficient car off the road.

First Things First

Before investing in a solar hot-water heater, it's important to first pick the low-hanging fruit of conservation and energy savings. Do things like turn down the thermostat of your existing hot-water heater (120° F is a good temperature for most people), wrap it with insulation, fix drips, and install flow restrictors and faucet aerators. By lowering your hot-water usage, you can reduce the size and cost of your solar water heater.

You'll also want to make sure your site has enough available sunshine to meet your needs efficiently and economically. In a nutshell, that means you should have decent southern exposure. To get more exact than that, contact a solar equipment dealer who can perform a solar site analysis for you or show you how to do your own. The Department of Energy's Energy Efficiency and Renewable Energy Clearinghouse (EREC 1-800-DOE-EREC; www.eren.doe.gov) is also a good source for information.

Basic Options

Over the years a number of different approaches have developed for solar water heaters. But in the most general sense, there are only two types: active and passive -- those that require nonsolar energy to work, and those that don't.

A passive solar design works without any pumps or electric components. This makes them less vulnerable to mishaps, easier to maintain, and possibly longer-lasting than active systems. The simplicity also tends to mean they are less expensive than active systems, but the tradeoff is they are typically less efficient.

Batch Heaters

The simplest passive systems are batch heaters (also sometimes referred to as "breadboxes" or integrated collector systems). They consist of one or more black tanks placed horizontally or vertically in a large, well-insulated collector box. The heaters can be placed on the roof, on the south wall of the house, or on the ground near the house (again, with good southern exposure). Some people have incorporated batch heater systems in the design of an attached sunspace which makes the system less conspicuous.

The water-filled tanks are protected by insulated glazed glass to let in sunlight, which warms the water and also holds in heat. Some boxes include reflectors to increase the solar radiation and the tanks may be made of especially absorbent materials. Water enters at the bottom of the collector, and solar-heated water is drawn off the top to your inside water tank as it is needed. Essentially, the sun preheats your water for you.

Batch heaters are relatively inexpensive (from $1000 to - $2000) and in warm climates can provide year-round hot water. But in the Northeast batch heaters are only truly effective in the summer. Then, they should, as EREC's Paul Hesse says, "meet all the modest, warm- to hot-water requirements [of a family]." Since batch heaters must be protected from freezing when temperatures drop below 32° F, however, once the weather becomes consistently cold, they should be drained.

Given their limited usage in New England, batch heaters make most financial sense if you can build one yourself. Because of their simplicity, a reasonably handy do-it-yourselfer could build and install a passive hot-water system for under $400. Be aware, though, that water-filled tanks are very heavy. Don't consider placing them on a roof unless it's strong enough to hold that kind of load.

More Complicated -- and Effective -- Systems

While the simplicity of a batch heater is appealing, its limitations are not. Since the holding tanks of a batch heater are large, it can take a whole day of sunshine to produce suitable water temperatures. And once that water is hot, you need to use it before the poorly insulated tank loses that warmth to the cool night air. This shows why if you want to take full advantage of generating hot water from the sun, it's better to separate the collector from the storage tank. Like other divisions of labor, this optimizes both functions. Leaving the collectors exposed to the sun and bringing the tank inside and insulating it well increase overall efficiency and year-round effectiveness.

Other benefits come from separating the collector and the storage tank. First, you can expand the collector's surface area in relation to the amount of water heated, allowing it to heat water much faster. Most collectors consist of thin tubes made of heat absorbent materials so the water in them will heat as quickly as possible. Second, by designing a storage tank that keeps the hottest water separated from the coldest, you can use the hottest water first, making it available sooner than waiting for a whole tankful to get warm. These advantages are especially important in freezing climates, where keeping your tank and piping indoors prevents it from freezing during cold weather.

Thermosiphon Systems

A thermosiphon system is a passive setup that relies on a phenomenon known as natural convection (warm water rises) to circulate H2O through the collectors and to the tank. In this arrangement the tank must be above the collector. When water in the collector heats up, it becomes lighter and rises naturally into the tank above. As this happens, cooler water in the tank flows down pipes to the bottom of the collector, creating circulation throughout the system. The storage tank is attached to the top of the collector so that thermosiphoning can occur.

These systems are reliable and relatively inexpensive ($2,000 to $3,000 installed), but according to Bill Waldron of Suntech Solar in Deerfield, Mass., who has been installing solar hot-water systems in New England for 15 years, thermosiphon systems aren't appropriate for cold climates. When temperatures drop below 32° F, the water in the system freezes. Some thermosiphon systems address this problem by circulating an antifreeze solution through a heat exchanger (so the antifreeze solution doesn't contaminate the household water), but this isn't enough to make such setups freezeproof during extended cold spells.

Active Systems

Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but are also more efficient. Active systems are usually easier to retrofit than passive systems because their storage tanks don't need to be installed near the collectors. But because they use electricity, they won't function in a power outage (unless, of course, you're off the grid). Active systems range in price from about $5,000 to $11,000 installed.

Open-Loop Active Systems

Open-loop active systems use pumps to circulate household water through the collectors. This design is efficient and lowers operating costs but isn't appropriate if your water is hard or acidic because scale and corrosion quickly disable the system.

Unless you live in a warm climate like Hawaii, open-loop systems must protect against the water that passes through the collectors from freezing. One way to do this is through what's called a recirculation system. Recirculation systems use a whole-system pump to circulate warm water from storage tanks through the collectors and any exposed piping when temperatures approach freezing. This design, however, isn't appropriate for cool or cold climates, since frequently needing to circulate warm water wastes electricity and stored heat. Also, if you lose power, the pump won't work and the system will freeze (which, yes is catastrophic as far as the system goes).

Closed-Loop Active Systems

Closed-loop active systems, which are appropriate for colder areas, pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Double-walled heat exchangers prevent contamination of household water. While glycol antifreeze systems provide good freeze protection, they're a bit more expensive to buy and install, and the glycol should be checked annually and--depending on glycol quality and system temperatures--changed every 3 to 6 years,. Drainback systems use distilled water as the heat-transfer fluid in the collector loop. A pump circulates the water through the collectors. Gravity drains the water to the storage tank and heat exchanger; there are no valves to fail. When the pumps are off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.

The pumps in solar water heaters have low power requirements, and some companies now include direct current (DC) pumps powered by small solar-electric (photovoltaic, or PV) panels. PV panels convert sunlight into DC electricity. Such systems cost nothing to operate and continue to function during power outages.

Sizing Your System

Just as you have to choose a 30-, 40-, or 50-gallon (114-, 151-, or 189-liter) conventional water heater, you need to select the right size solar water heater to install. Sizing a solar water heater involves determining the total collector area and the storage volume required to provide 100 percent of your household's hot water during the summer. Solar-equipment installers use worksheets or special computer programs to assist you in calculating how large a system you need.

Solar storage tanks are available in 50- to 120-gallon sizes. An 80-gallon tank is usually fine for a family of four. According to Home Power's Ken Olson, in New England, for every .75- gallon of tank capacity you need 1 square foot of collector. So a New Hampshire household of four would need about 100 square feet of collector.

One of the great things about installing a solar hot-water heater is that it allows you to take a significant, beyond-the-ordinary step toward energy independence and reducing your environmental footprint without a major retrofit of your home and without needing the capital necessary to be completely off the grid. And as the years go by, your ecogreen hue will translate into greenbacks that accumulate in your savings account.